Paper sheet feed apparatus for offset printing machines and the like have been disclosed in the following U.S. Pat. Nos. 1,575,892; 2,085,592; 2,937,868; 2,986,392; and 3,991,997. All of these patents show sheet pickup devices which are fixed in nature. They cannot be adjusted to accommodate paper sheets of differing weights.
A problem arises oftentimes when paper sheets of different weights are to be fed one-by-one from a stack into an offset printing machine. The stacking apparatus of such a machine has a number of spaced tabs which overlie the leading edge of the upper sheet of the stack to prevent air which blows laterally against the leading edges of the sheets from elevating the upper sheet until it is to be picked up by the suction pickup device. The air helps to prevent the sheets from sticking together.
For relatively lightweight paper, such as 10 to 12 lb. paper, the sheets are so flexible that, when the upper sheet is picked up, it is sometimes held back by the tabs and is stripped of the pickup devices. When this occurs, the sheet is not fed to the pressure and blanket rollers of the machine. Also a lightweight sheet becomes undulating or wavy at its leading edge when picked up; thus, when it is urged forwardly by the pickup devices, it does not feed properly between the blanket and pressure rollers. This causes the sheets to become jammed in the machine and operation of the machine must be halted to remove the sheet.
Because of the foregoing problems, a need has arisen for an improved sheet feed mechanism which can be used with sheets of many different weights so that such sheets can be fed equally well into an offset printing machine without interruption or without becoming jammed in the machine.